This invention relates generally to porous iron ore pellets, and more particularly to iron ore pellets, which are, in addition to possession of the properties which are required for a burden material of a blast furnace, improved in particular in reducibility, properties at high temperatures such as softening and sticking, repose angle, non-flowability into a coke layer, compressive strength, and a process for producing such iron ore pellets.
In a case where a large quantity of pellets is charged into a blast furnace, it is considered to be difficult to stabilize the blast furnace operation at a high level as compared with a case using sinter. This tendency is gathered to be attributable to the spherical shape, small repose angle due to high density and softening and sticking properties of the pellets. The pellets are apt to segregate at the center of the furnace when charged through the furnace top. In addition, the pellets which are in contact with adjacent pellets only at one point are inferior in the power of retaining a layer and therefore the pellet layer easily disintegrate in the stage of burden descending disturbing the distribution of burden materials and of gas flow. Further, the pellets are inferior to sinter in softening and sticking properties.
Various studies have thus far been made in an attempt to obtain pellets of improved properties and shape which ensure stable furnace operation even when the pellets are used in a large amount. For example, self-fluxing pellets with improved reducibility and physical strength and MgO-containing self-fluxing pellets with improved softening and sticking properties have been proposed and put into practice.
The MgO-containing self-fluxing pellets have relatively good reducibility at high temperatures but not as good as that of sinter for the reasons discussed below.
As the pellets descend in a blast furnace, they are subjected to higher temperatures undergoing reduction with a gas which diffuses into fine pores of the pellets, reducing iron oxide into FeO and then into Fe. In this instance, a slag containing FeO and having a low melting point is produced within the pellets in the high temperature zone. The low melting point slag produced in the high temperature zone exudes and clogs the fine pores of the pellets, causing the phenomenon which is generally referred to as "retardation of reduction".
With the self-fluxing pellets containing MgO, the slag contains MgO and thus has a higher melting point so that the exudation of the slag and clogging of pores are lessened. However, the adverse effects of the slag is unignorable since the pores have very small diameters.
The clogging of pores hinders the reduction from proceeding in a sufficient degree within the pellets. Upon entering the high temperature zone, the pellets which bear the FeO containing slag soften and contract to increase the permeability resistance of the iron ore pellet layer and at the same time the pellets melt and boil by direct contact with a coke layer of high temperature, imparing the permeability of the coke layer and hindering smooth operation of the furnace.
The reducibility of the pellets (the so-called retardation of reduction) in the high temperature zone can be improved effectively by increasing the porosity and pore diameters of the individual pellets. The increase of the porosity of iron ore pellets can contribute to improvement in reducibility in the regions leading to the high temperature zone, namely, to the decrease of the amount of FeO in the high temperature zone, while the increases in pore diameter contribute to the improvement of reducibility and to lessening the clogging of pores by the low melting point slag.
The porosity and pore diameter can be increased by:
(a) Lowering the firing temperature; and PA1 (b) Adding a combustible material. PA1 (1) The pellets are susceptible to cracking and have a low compressive strength due to a large FeO content; PA1 (2) The use of a high carolific material causes excessive slagbonding and retards reduction after FeO; and PA1 (3) The pore diameters are too large to retain a suitable compressive strength.
When the firing temperature is lowered, the porosity is increased as indicated by curve 4 of FIG. 2 but the pore diameter becomes smaller, with a lower physical strength due to insufficient sintering of the internal structure. Therefore, the pellets soften and contract to a considerable degree unsuitable for practical use.
A method for producing porous pellets by adding a combustible material is disclosed, for example, in Japanese Laid-Open Patent Specifications 119403/1977 and 10313/1978, each using a material combustible at a relatively high temperature. The pellets obtained by these methods have pores of large diameters but are unsuitable for actual use in a blast furnace for the following reasons.
For subsequent pelletization, the combustible material to be blended into iron ore should be ground into a particle size smaller than 2 mm. When the combustible material is admixed in an amount of 0.5 to 8% by weight, particles of about 2 mm in diameter are apt to form cores in the pelletizing stage. Therefore, in a case where the combustible material contains coarse particles in a great proportion, core-like particles are abnormally increased during the pelletizing operation in a pelletizer (e.g., disc or drum type pelletizer), causing a shortage of finer particles which are necessary for the growth of the cores, namely, hindering the growth of pellets or sometimes making the pelletization almost impossible. Even if somehow pelletized into desired sizes, the resulting pellets bear coarse particles on the outer peripheral surfaces or contains dumplings of agglomerated coarse particles which lower the productivity of green pellets of appropriate sizes or cause various problems in the subsequent firing stage. For example, the coarse core-like particles easily come off the pellet surfaces and the dumplings of agglomerated coarse particles readily disintegrate in the firing stage, causing clogging of the grate by deposition or production of an increased amount of dust which is deleterious to the efficiency of operation and the service life of the firing equipment. In addition, the coarse particles lower the yield to a considerable degree.
Further, the existence of coarse particle makes it difficult to admix the combustible material uniformly with iron ore and to maintain a uniform porosity over the individual pellets. Another difficulty attributable to coarse particles is that drop resistance of green pellets which are blended with the combustible material including coarse particles is as low as 50 to 60% of that of green pellets which the combustible material is not added. Such a large fall of the drop resistance is considered to be attributable solely to the inclusion of coarse particles in the pellets. As a result, the green pellets easily crack or break into smaller pieces even when conveyed from a pelletizer to a firing apparatus, reducing the yield of pellets to a considerable degree.
In order to solve these problems, there should be employed a combustible material which contains coarse particles in as small a proportion as possible and which is ground to have a grain or particle size smaller than 2 mm, preferably, smaller than 0.5 mm.
The above-mentioned combustible materials are generally extremely low in crushability, for example, the grinding work index Wi (JIS M 4002) of sawdust is as high as about 600 kwh/t in contrast to Wi of iron ore which is 6-25 kwh/t or to Wi of petroleum coke which is about 70 kwh/t. Moreover, there is a possibility of dust explosion when a combustible material alone is forcibly pulverized and it is difficult to completely preclude the danger of explosion by employing ordinary explosion-proof measures.